After you finish stage 4 a timer expires, asking you to insert another credit.
The timer is currently signalled by that long beep the game does, no idea if there are 7 segment LEDs displaying current time or it's supposed to be displayed on video, the little "credit" message that pops up quickly after a reset is suspicious too.

You also need to press a button after inserting the credit, which I imagine is a start.
You can start doing it in 0.189